1. Field of the Invention
The present invention relates to an alkaline rechargeable battery whose anode comprises a particulate (or a powder) of an alloy capable of reversibly storing and releasing hydrogen as a main component and a process for the production of said rechargeable battery. More particularly, the present invention relates to an alkaline rechargeable battery having an anode formed using an alloy particulate capable of reversibly storing and releasing hydrogen as a main component, said anode having an excellent electrode activity and a high active-material utilization efficiency, excelling in resistance to overcharge and having a prolonged life time, and said rechargeable battery having excellent charge-and-discharge characteristics, excelling in resistance to overcharge, having a prolonged cycle life, and being capable of being provided at a reasonable cost. The present invention includes a process for the production of said rechargeable battery.
The term “particulate” in the present invention includes a powder and comprises separate particles having a given average particle size.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect due to an increase in the content of CO2 gas in the air has been predicted. For instance, in thermal electric power plants, thermal energy obtained by burning a fossil fuel is being converted into electric energy, and along with burning of such fossil fuel, a large amount of CO2 gas is being exhausted in the air. Accordingly, in order to suppress this situation, there is a tendency of prohibiting the new establishment of thermal electric power plants. Under these circumstances, so-called load leveling practice has been proposed in order to effectively utilize electric powers generated by power generators in thermal electric power plants or the like, wherein a surplus power unused in the night is stored in rechargeable batteries installed at general houses and the power thus stored is used in the daytime when the demand for power is increased, whereby the power consumption is leveled.
Separately, in recent years, gasoline-fueled vehicles also have been becoming an issue because they exhaust air polluting substances such as CO2, NOx, hydrocarbons and the like. On the other hand, electric vehicles which are driven by virtue of electricity stored in the rechargeable batteries provided therein without exhausting such air polluting substances have attracted the public attention, and research and development have been vigorously conducted in order to put such electric vehicles to practical use. Along with this, there is an increased demand for developing a high performance rechargeable battery having a high energy density and a prolonged cycle life and which can be provided at a reasonable cost.
Besides, there is also an increased demand for developing a miniature, lightweight, high performance rechargeable battery usable as a power source for portable instruments such as small personal computers, word processors, video cameras, and cellular phones.
In order to comply with these demands, research and development have been vigorously conducted on nickel-metal hydride rechargeable batteries in which a cathode comprising a nickel hydroxide as a cathode active material and an anode comprising an alloy capable of reversibly storing and releasing hydrogen (this alloy will be hereinafter referred to as “hydrogen storage alloy”) as an anode active material are used. And some of these nickel-metal hydride rechargeable batteries have been put to practical use.
Incidentally, metals can be roughly categorized into two groups, i.e., a group of exothermic type metals which can exothermicaly react with hydrogen to produce a stable hydride and a group of endothermic type metals which can endothermicaly react with hydrogen but do not have a chemical affinity with hydrogen under normal condition. As typical specific examples of the exothermic type metal, there can be mentioned alkali metals, alkaline earth metals, rare earth metals, and transition metals belonging to groups 4 and 5 of the periodic table in accordance with the classification method of IUPAC (International Union of Pure and Applied Chemistry) enacted in 1990. As typical specific examples of the endothermic type metal, there can be mentioned transition metals belonging to groups 6 to 9 and also to group 10 (excluding Pd) of aforesaid periodic table.
Now, there are known various hydrogen storage alloys capable of reversibly storing and releasing hydrogen, comprising a combination of a given exothermic type metal and a given endothermic type metal. The exothermic metal in the hydrogen storage alloy functions to strongly bond with hydrogen, and the endothermic metal, specifically Fe, Co, or Ni in the hydrogen storage alloy functions as a catalyst to dissociate molecular hydrogen deposited thereon into hydrogen atoms. Thus, upon the preparation of such hydrogen storage alloy, by adjusting the kinds of metal elements to constitute the alloy and controlling the composition ratios of the metal elements, it is possible to obtain a desired hydrogen storage alloy having an equilibrium hydrogen pressure which matches with given use purposes.
The hydrogen storage alloys which presently have been used in the anodes of the rechargeable batteries are mostly Mischmetal series alloys (comprising a mixture of rare earth metals) represented by Mm(Ni—Co—Mn—Al)5 alloys (with Mm being Mischmetal). Besides, there have been proposed Laves phase alloys such as Zr—Ti—Ni—Mn—V—Cr—Co alloy and the like, and some of them have been put to practical use as an anode material of a rechargeable battery. Separately, studies have been made of magnesium-nickel series alloys such as Mg2Ni alloy, Mg—Ni alloy and the like and also of bcc (body-centered cubic structure) type solid solution alloys such as Ti—V—Ni alloy and the like with respect to the possibility of their use as an anode material of a rechargeable battery.
Incidentally, in comparison with the Mischmetal series alloy, the Laves phase alloy is capable of storing hydrogen in a larger amount and is relatively stable to an alkali electrolyte solution. Therefore, the Laves phase alloy has been considered to be promising to use as an anode material of a rechargeable battery. However, the Laves phase alloy has drawbacks such that Zr, Ti and V which are the principal elements to constitute the Laves phase alloy and which are belonging to the foregoing exothermic metals are more likely to react with oxygen in the air to form a solid oxide and because of this, a particulate of such alloy is covered by a surface layer comprising such solid oxide in general. Thus, a rechargeable battery having an anode formed using such Laves phase alloy is extremely inferior in terms of the reaction activity at the initial stage of the charge-and-discharge cycle. In order to eliminate this problem, it is necessitated that the discharge capacity of the rechargeable battery is increased to a prescribed value by way of an initial activation treatment. The initial activation treatment includes a treatment wherein the rechargeable battery is subjected to a heat treatment prior to performing the initial charging for the rechargeable battery and a treatment wherein the rechargeable battery is subjected to a treatment of repeating a cycle of charging and discharging about 10 times.
This situation of the Laves phase alloy is similar also in the case of the body-centered cubic structure type solid solution alloy. Particularly, the body-centered cubic structure type solid solution alloy has a function to store hydrogen in a large amount as well as the Laves phase alloy. Thus, the body-centered cubic structure type solid solution alloy has been expected to be usable as a high capacity anode material in a rechargeable battery. However, the body-centered cubic structure type solid solution alloy has drawbacks such that Ti and V which are the principal elements to constitute the body-centered cubic structure type solid solution alloy are more likely to react with oxygen in the air to form a solid oxide and because of this, a particulate of such alloy is covered by a surface layer comprising such solid oxide in general, as well as in the case of the Laves phase alloy. Thus, for a rechargeable battery having an anode formed using such body-centered cubic structure type solid solution alloy, it is also necessitated that the rechargeable battery is subjected to such initial activation treatment as above described, in order to improve the performance.
About one week is generally required to complete the foregoing initial activation treatment in order to improve the performance of such rechargeable battery as above described. This raises the production cost of the rechargeable battery. This situation interrupts to put the above-mentioned hydrogen storage alloys to practical use as an anode material of a rechargeable battery, although these hydrogen storage alloys have a pronounced advantage in that they are capable of providing an electrode (an anode) having a high capacity usable in a rechargeable battery.
Incidentally, in a sealed type rechargeable battery having an anode comprising a hydrogen storage alloy and which is housed in a battery housing having a safety vent, there is generally adopted a system in that the capacity of the anode is made to be greater than that of the cathode so that oxygen gas generated from the cathode at last stage of charging is absorbed by the anode to reduce into water. For the sealed type rechargeable battery, there is a disadvantage such that when the hydrogen storage alloy constituting the anode is inferior in terms of the initial activity, hydrogen gas is liable to generate from the anode at the initial state of the charge-and-discharge cycle, and when said hydrogen gas is generated, the inner pressure of the rechargeable battery is increased to open the safety vent of the battery housing, where there is an occasion in that the electrolyte solution in the rechargeable battery is flied off to shorten the lifetime of the rechargeable battery. There is also a disadvantage such that it is difficult to form a proper discharge reserve (a surplus capacity in a discharged state provided utilizing an irreversibly reacting component of the cathode at an initial stage of performing charging in order to prevent the anode from being polarized) in the anode and because of this, when the rechargeable battery is discharged at a high rate, the capacity of the anode is liable to greatly decrease.
In order to prevent these problems from occurring, it is necessary to enlarge the ratio between the capacity of the anode and that of the cathode. However, when the ratio between the capacity of the anode and that of the cathode is enlarged, there will be a disadvantage such that the energy density of the rechargeable battery is diminished.
In order to eliminate such problems as above described, there has been proposed a method of treating a powdery hydrogen storage alloy in an intense alkali solution maintained at elevated temperature prior to using it in the formation of an anode of a rechargeable battery. For instance, in Industrial Research Institute Journal No. 391, page 32, published in 1997 by Osaka Industrial Research Institute of Agency of Industrial Science and Technology (hereinafter referred to as Document 1), there is described a method wherein a powdery Zr series C15 type Laves phase alloy as a hydrogen storage alloy is treated by boiling it in a potassium hydroxide aqueous solution of 6M. In Document 1, there is also described that according to this method, oxide coats of Ti and Zr on the surface of the powdery alloy are dissolved and removed while Mn and V as soluble materials contained in the powdery alloy are dissolved and removed, where a coat with a high Ni-content is formed on the surface of the powdery alloy, and when the powdery alloy thus treated is used in the formation of an anode for a rechargeable battery, the anode excels in the initial activity. However, the method disclosed in Document 1 has such disadvantages as will be described in the following. A complicated process including a washing step with water and a drying step is required to be performed after the treatment by the potassium hydroxide aqueous solution in order to obtain a desirable powdery hydrogen storage alloy. This serves to raise the production cost of the product. In addition, the surface of the powdery alloy obtained in accordance with the method disclosed in Document 1 is liable to oxidize and therefore, when the powdery alloy is allowed to stand in the atmospheric air over a long period of time, the surface of the powdery alloy is deactivated. Thus, it is necessitated that immediately after the treatment by the potassium hydroxide aqueous solution, the powdery alloy is subjected to the formation of the anode.
Separately, there has been proposed a method wherein a powdery hydrogen storage alloy is mixed with a powdery nickel to obtain a mixture and the mixture is subjected to a treatment with the application of mechanical energy, whereby a hydrogen storage alloy particulate deposited with a nickel particulate on the surface is obtained. For instance, in GRINDING No. 41, p.p. 42-43, published in 1997 by Fine Particle Engineering Research Institute of Hosokawa Micron Kabushiki Kaisha (hereinafter referred to as Document 2), there is described a method wherein nickel fine powder is deposited on the surface of a powdery hydrogen storage alloy of ZrMn0.6V0.2Cr0.2Ni1.2 by way of a mechano-fusion treatment. Document 2 also describes that according to this method, a Ni-diffused layer is formed on the alloy surface to provide an increase in the specific surface and an improvement in the electrode activity and the initial activity of the electrode and the high rate discharge characteristics are improved. Thus, when the method described in Document 2 is adopted in the formation of an anode for a rechargeable battery, having an active material layer comprising such powdery hydrogen storage alloy on a collector, it is considered that there would be provided an effect in that the conductivity between particles of the powdery alloy and that between the powdery alloy and the collector and an effect in that the powdery alloy functions as a catalyst in the battery reaction. However, in the experimental studies by the present inventors, no distinguished effect has been recognized with respect to the function as the catalyst, and the number of the charge-and-discharge cycles required for the initial activation treatment of the rechargeable battery has slightly diminished. For the reason for this, it is considered such that the nickel fine particles deposited on the powdery alloy surface are present through a solid oxide coat of Zr or the like previously formed on the powdery alloy surface, the powdery alloy does not effectively function as the catalyst.
There has been proposed a method of removing an oxide coat formed on the surface of a powdery hydrogen storage alloy while nickel particles are deposited on the powdery alloy surface. For instance, Japanese Laid-open Patent Application No. 9(1997)-312157 (hereinafter referred to as Document 3) discloses a method wherein a powdery hydrogen storage alloy is subjected to a reduction treatment with the use of hydrogen gas or it is subjected to an etching treatment with the use of an aqueous solution of hydrofluoric acid to remove an oxide coat formed on the surface of the powdery hydrogen storage alloy and nickel fine particles are deposited on the surface of the treated powdery hydrogen storage alloy by way of a ball mill treatment or a mechano-fusion treatment. However, the method disclosed in Document 3 has such disadvantages as will be described in the following. The oxide coat formed on the powdery hydrogen storage alloy cannot be sufficiently removed by the reduction treatment using the hydrogen gas. According to the etching treatment using the hydrofluoric acid aqueous solution, although the oxide coat can be removed, the necessary elements of the powdery hydrogen storage alloy are dissolved and removed upon the etching treatment. And in the case where the etching treatment using the hydrofluoric acid aqueous solution is adopted, a complicated process including a washing step with water and a drying step is required to be performed after the etching treatment in order to obtain a desirable powdery hydrogen storage alloy. This serves to raise the production cost of the product. In addition, the surface of the powdery hydrogen storage alloy obtained is liable to oxidize and therefore, when the powdery hydrogen storage alloy is allowed to stand in the atmospheric air over a long period of time, the surface of the powdery hydrogen storage alloy is deactivated.
Japanese Laid-open Patent Application No. 7(1995)-37582 (hereinafter referred to as Document 4) discloses a method wherein a mixed powder of a powdery hydrogen storage alloy of ZrMn0.6V0.2Cr0.1Ni1.2, a powdery nickel hydroxide is admixed with a powdery calcium as a reducing agent in an amount exceeding the amount of the powdery nickel hydroxide, the resultant is stirred, followed by subjecting to a washing treatment with water, to obtain a mixture comprising a black nickel powder and the powdery hydrogen storage alloy. However, in the mixture obtained in accordance with the method disclosed in Document 4, the nickel powder is present merely in a mixed state and therefore, it does not have a remarkable hydrogen activity. And in the method disclosed in Document 4, the calcium and calcium hydroxide in the mixture are removed by the water-washing treatment. This makes the process complicated to raise the production cost of the product.
Japanese Laid-open Patent Application No. 8(1996)-69795 (hereinafter referred to as Document 5) discloses a method for the production of a hydrogen storage alloy electrode having an improved initial activity and which is usable as the anode of a rechargeable battery. Specifically, Document 5 discloses a method wherein a mixed powder of a powdery hydrogen storage alloy of Zn(V0.1Ni0.64Mn0.38)2.1 and a powdery magnesium as a reducing agent is subjected to a rolling treatment together with a porous body formed of an alkali-corrosive resistant metal to form an electrode form and the electrode form is subjected to a heat treatment at a temperature of 700 to 1000° C., which is higher than the melting point of the reducing agent, in an inert gas atmosphere, whereby a hydrogen storage alloy electrode having an improved initial activity is obtained. Document 5 describes that according to this method, an oxide layer formed on the surface of the powdery hydrogen storage alloy is reduced into a metal. However, in the method disclosed in Document 5, the magnesium as the reducing agent and the hydrogen storage alloy are reacted at such high temperature as above described, where the elements constituting the hydrogen storage alloy are partly alloyed with the magnesium to cause a change in the composition of a surface region of the hydrogen storage alloy. Therefore, although the initial activity of the hydrogen storage alloy electrode is improved, the maximum discharge capacity of a rechargeable battery in which the hydrogen storage alloy electrode is used as the anode is reduced in practice.
U.S. Pat. No. 6,040,087 hereinafter referred to as Document 6) discloses a powdery material usable as the anode of a rechargeable battery, comprising a core comprising a powdery hydrogen storage alloy, a layer formed to cover the surface of the core, comprising a transition metal oxide layer or a transition metal oxide layer incorporated with aluminum or silicon; and a metal element having a function to make hydrogen to be in an active state, dispersed on the surface of said layer. Document 6 describes that the powdery material has an improved discharge capacity and excellent overcharge-resistant characteristics. However, for a rechargeable battery in which the powdery material is used as the anode, there is still a subject to be improved with respect to shortening the time required for the initial activation treatment.